The oxygenation of polyunsaturated fatty acid to prostaglandins is catalyzed by the membrane bound enzyme complex prostaglandin synthetase, which is widely distributed in animal tissue. We have recently found that during the early stages of oxygenation a variety of xenobiotics, including the carcinogenic hydrocarbon benzo(a)pyrene, are cooxygenated. Hydroperoxide intermediates of prostaglandin biosynthesis appear to be responsible for this cooxygenation by interacting with an unidentified microsomal enzyme. We propose to investigate the mechanism of the enzyme catalyzed hydroperoxide dependent oxidation with 180 tracer studies. We have synthesized hydroperoxides labeled with 180 in both peroxy oxygens. We will use this material to determine whether the oxygen incorporated into cooxidized xenobiotics is transferred directly from the hydroperoxides. Complementary experiments will be performed in an 1802 atmosphere and in the presence of H2 180. These experiments will provide important mechanistic information regarding prostaglandin synthetase dependent cooxidation.